Many attempts have been made in an attempt to simplify differential pressure servomotors. In U.S. Pat. No. 3,106,873 a one piece plastic backing plate and hub was introduced to reduce the number of parts required to manufacture such pressure differential servomotors. A pressure differential developed across a diaphragm acts on the plastic backing plate to transmit an output force to a push rod retained in an axial bore in the hub. After an extended period of time of maximum operation, it has been possible to produce a fracture of the plastic in the area of a passageway adjacent a reaction means associated with the push rod through which the output force is transmitted. The fracture has been determined to be caused by fatigue in the plastic resulting from repeated internal stressing primarily caused by the radial forces produced by the reaction means.
Another attempt at simplification of pressure differential servomotor is disclosed in U.S. Pat. No. 3,754,450 wherein a plastic hub member is attached to a metal backing plate. In this design, in addition to the passageway, a slot is provided for the insertion of a lug which holds the control valve in the hub. However, after repeated maximum development of the operational pressure differential this hub has also shown small fractures even though additional supporting ribs were added in the passageway area.